> Who says your filesystem isn't compressing data under you?

Me, because we don't use transparent compression anywhere.

> Actually, who says the disk itself isn't?

I'm not very sure about that, to be honest. SSDs do that to prolong their life, but the saved space doesn't return as more space on FS level. It just returns as endurance, which is more important in my case.

So, disk's transparent compression doesn't mean very much from a sysadmin/OS operations perspective.